1. Field of the Invention
The present invention relates to an open-air optical communication system that avoids signal degradation due to attenuation and scattering.
2. Description of the Prior Art
Open-air optical communication systems have been available for decades and cover the range from single RS-232 units operating at 1200 bps to high-speed ATM units capable of live video broadcasts. While all conventional optical communication systems operate with weather related constraints, continuing advances in bandwidth improvement, cost reductions, and the minimization of atmospheric affects have aided in bringing optical communication systems into the mainstream of communication products available to the telecommunications engineer. Exemplary developments in this regard are reported in the following recent technical papers: T. Wang, G. R. Ochs, and S. F. Clifford, A Saturation-resistant Optical Scintillometer to Measure C.sub.n.sup.2, J. Opt. Soc. Am. 68, 334 (1978); S. F. Clifford, G. R. Ochs, and R. S. Lawrence, "Saturation of Optical Scintillation by Strong Turbulence", J. Opt. Soc. Am. 64, 148 (1974); R. S. Lawrence, G. R. Ochs, and S. F. Clifford, "Measurements of Atmospheric Turbulence Relevant to Optical Propagation", J. Opt. Soc. Am. 60, 826 (1970); G. R. Ochs, and Ting-I. Wang, "Finite Aperture Optical Scintillometer for Profiling Wind and C.sub.n.sup.2 ", Appl. Opt., vol. 17, No. 23, 3774-3778 (1978); R. M. Gagliardi and S. Karp, Optical Communications, John Wiley & Sons, Inc., New York, 1995; C. P. Primmerman, et. al., "Atmospheric-Compensation Experiments in Strong-Scintillation Conditions", Applied Optics 34, No. 12, p. 2081-2088, 1995; and J. H. Shapiro, "Imaging and Optical Communications Through Atmospheric Turbulence", Laser Beam Propagation in the Atmosphere, J. W. Strohbehn, Ed., Springer-Verlag, New York, p.171-222, 1978.
Despite significant advances in the field of open-air optical communication, the development of such systems has been hampered by certain basic, underlying effects upon open-air optical communication systems that are unique to this type of communication. Specifically, the atmospheric optical channel may be seen as clear air, or it may contain particles from dust, fog, mist, or precipitation. When a light beam passes through the atmosphere containing fog, rain, or other particles, both attenuation and scattering occur. A collimated beam broadens due to the scattering, thus resulting in losses in signal strength. During heavy fog or snow, the light beam is totally obscured. Under such conditions no light can be transmitted to the other end of the communication system so that the open-air communication channel is interrupted. Essentially, there is no simple solution to overcome the basic limitations imposed by the laws of physics.
However, even in clean air conditions, atmospheric turbulence-induced optical scintillation may severely affect the quality of optical communications systems. Atmospheric turbulence induced optical scintillation is particularly important to understand when designing wireless communication solutions using an optical device. The shimmering eddies seen above a hot surface and the twinkling of stars are examples of turbulence induced optical scintillation. Temperature gradients within and between these eddies cause refractive index changes on the light as it passes from a transmitter to a receiver through these eddies. These changes act as additional optical lenses that orient and refocus the optical beam. Most of the light intensity fluxuation that occurs is a result of the refraction of the beam of light. That is, it results from scattering rather than attenuation.